[llvm-lib] Ignore /SUBSYSTEM flag

[pzheng]

Summary

It's not clear what Microsoft's LIB.exe actually does based on the official
description of the flag (link below). We can probably ignore it for now.

https://docs.microsoft.com/en-us/cpp/build/reference/managing-a-library?view=msvc-170

[thieta]

LGTM.

[pzheng]

Thanks!

llvm/lib/ToolDrivers/llvm-lib/Options.td

 def help_q : Flag<["/??", "-??", "/?", "-?"], "">, Alias<help>;
 
 //==============================================================================
 // The flags below do nothing. They are defined only for lib.exe compatibility.
 //==============================================================================
 
 def ltcg : F<"ltcg">;
 def nologo : F<"nologo">;
+def subsystem : P<"subsystem", "">;